U.S. patent number 5,227,259 [Application Number 07/736,073] was granted by the patent office on 1993-07-13 for apparatus and method for locating and isolating failed cells in a battery.
This patent grant is currently assigned to Electric Power Research Institute, Inc.. Invention is credited to Francis L. Tanzella, Robert D. Weaver.
United States Patent |
5,227,259 |
Weaver , et al. |
July 13, 1993 |
Apparatus and method for locating and isolating failed cells in a
battery
Abstract
An apparatus and method for locating and electrically isolating
failed cells in a network of cells is disclosed. The apparatus
includes a device for sampling voltage and current levels of each
of the cells of the battery. Another device compares the sampled
voltage and current levels to voltage and current limits to
determine whether a cell has failed. If a cell has failed, control
circuitry produces an activation signal which is conveyed to a
switch associated with the failed cell. The activation signal
drives a heating element of the switch. The heating element, which
surrounds a fusible link of the switch, melts the fusible link. The
fused link congregates at the base of the switch. In the case of a
parallel network of cells, the fused link forms an open circuit,
while in the case of a series network of cells, the fused link
forms a bypass circuit. Consequently, the switch isolates the
failed cell from the good cells remaining in the battery.
Inventors: |
Weaver; Robert D. (Palo Alto,
CA), Tanzella; Francis L. (Newark, CA) |
Assignee: |
Electric Power Research Institute,
Inc. (Palo Alto, CA)
|
Family
ID: |
24958401 |
Appl.
No.: |
07/736,073 |
Filed: |
July 24, 1991 |
Current U.S.
Class: |
429/49; 429/61;
324/434; 429/90 |
Current CPC
Class: |
H01M
10/4207 (20130101); H01M 50/581 (20210101); G01R
31/396 (20190101); H01M 50/572 (20210101); Y02E
60/10 (20130101) |
Current International
Class: |
G01R
31/36 (20060101); H01M 2/20 (20060101); H01M
2/34 (20060101); H01M 10/42 (20060101); H01M
010/44 (); H01M 002/00 () |
Field of
Search: |
;429/90,61,49 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chaudhuri; Olik
Assistant Examiner: Nuzzolillo; M.
Attorney, Agent or Firm: Flehr, Hohbach, Test, Albritton
& Herbert
Claims
We claim:
1. An apparatus for locating and electrically isolating failed
cells in a network of cells within a battery, said apparatus
comprising:
means for determining the condition of said cells of said network
of cells of said battery so as to identify a failed cell, said
determining means including:
means for sampling the electrical properties of said cells of said
battery;
means for interpreting said electrical properties of said cells to
determine the condition of said cells; and
means for conveying an activation signal to said failed cell;
and
means for altering the electrical connection between said filed
cell and said network of cells of said battery, said altering means
being coupled and responsive to said determining means.
2. The apparatus of claim 1 wherein said sampling means is a
sequential sampling multiplexer.
3. The apparatus of claim 1 wherein said interpreting means
includes a microprocessor.
4. The apparatus of claim 1 wherein said switching means includes a
microprocessor coupled to said failed cell.
5. The apparatus of claim 1 wherein said altering means includes a
switch which alters the electrical connection between said failed
cell and said network of cells of said battery.
6. The apparatus of claim 5 wherein said switch includes a fusible
link.
7. The apparatus of claim 6 wherein said fusible link is melted by
a coil.
8. The apparatus of claim 7 wherein said fusible link is positioned
within a sealed housing.
9. The apparatus of claim 8 wherein said fusible link is positioned
within a wetting tube which is positioned within said housing.
10. The apparatus of claim 6 wherein said fusible link includes a
metal coating.
11. An apparatus for locating and electrically isolating failed
cells in a network of cells within a battery so as to prevent a
failed cell from interfering with the operation of functional
cells, said apparatus comprising:
means for sampling the electrical properties of said cells of said
battery;
means for interpreting said electrical properties of said cells to
determine the condition of said cells;
means for generating an activation signal to a cell in response to
said interpreting means; and
means for conveying said activation signal to a switch positioned
at said failed cell, said switch including means for melting a
fusible link associated with said switch, whereby the melted
fusible link results in a connection isolating said failed cell
from said network of cells so as to prevent said failed cell from
interfering with the operation of functional cells in said
batter.
12. The apparatus of claim 11 wherein said sampling means includes
a sequential sampling multiplexer.
13. The apparatus of claim 11 wherein said comparing means and said
switching means include a microprocessor.
14. The apparatus of claim 11 wherein said switch includes a sealed
housing.
15. The apparatus of claim 14 wherein said fusible link is
positioned within a wetting tube which is positioned within said
housing.
16. The apparatus of claim 11 wherein said fusible link is formed
of aluminum and includes a metal coating.
17. A method for locating and electrically isolating failed cells
in a network of cells within a battery, said method comprising the
steps of:
sampling the electrical properties of said cells of said
battery;
interpreting said electrical properties of said cells to determine
the condition of said cells;
conveying an activation signal to said failed cell in response to
said interpreting step; and
altering the electrical connection between said failed cell and
said network of cells of said battery in response to said
activation signal.
18. The method of claim 17 wherein said altering step includes
heating a fusible link of a switch associated with said failed cell
so as to alter the electrical connection between said failed cell
and said network of cells of said battery.
Description
BRIEF DESCRIPTION OF THE INVENTION
This invention generally relates to an apparatus and method for
successfully operating a battery notwithstanding the presence of a
failed cell within the battery. More particularly, it relates to an
apparatus and method for locating and then isolating a failed cell
without interrupting the electrical environment of the remaining
cells in the battery.
BACKGROUND OF THE INVENTION
Batteries for the storage of electrical energy are known in the
art. They are used in a wide variety of applications. The
applications vary from space satellites, which usually will
represent relatively small energy storage requirements, through
electric vehicles which represent energy storage at the 50
kilowatt-hour level, to electric utility load leveling batteries
which will require 100 megawatt-hour levels of energy storage. The
number of cells associated with a battery may vary from 10 to over
2,000,000. As the complexity of a battery increases, it is
increasingly necessary to provide means for locating and isolating
a failed cell within the battery. Otherwise, a failed cell can
deplete energy from good cells or actually cause physical damage to
good cells.
The network of cells in a battery may be connected in series or in
parallel. A failed or failing cell has a substantially adverse
impact on the remaining good cells. For cells in parallel, a cell
which has failed in such a manner as to create a short-circuit can
drain the capacity of all of its parallel network members.
Similarly, for cells in series, a cell which has failed in such a
manner as to become non-conductive, or to become an open-circuit,
removes from access the capacity of its undamaged series members.
The failed cell problem is compounded by the fact that cells
generally fail in a random manner, thus preventing any solution
based upon a predicted location of a failed cell or the predicted
kind of failure of the cell.
In the prior art, these problems have been addressed through the
utilization of fuses. The successful use of fuses requires excess
current flow through the cell as a result of cell failure. The
excess current must be sufficient to melt the fusible link of the
fuse. This approach is unreliable since a failed cell need not
result in current of sufficient magnitude to cause the desired
fusing action.
OBJECTS AND SUMMARY OF THE INVENTION
It is a general object of the present invention to provide an
apparatus and method for ongoing operation of a battery despite the
presence of failed cells.
It is a more particular object of the present invention to detect
the presence of a failed cell in a battery.
It is a related object of the present invention to provide an
apparatus and method for isolating failed cells.
It is another object of the present invention to isolate failed
cells without reliance on a fuse.
It is another object of the present invention to provide a
mechanism for isolating failed cells regardless of their position
within the electrical network.
It is still another object of the present invention to isolate
failed cells in an environment without gravity.
These and other objects are achieved by an apparatus and method for
locating and electrically isolating failed cells in a network of
cells of a battery. The apparatus and method prevent a failed cell
from interfering with the operation of functional cells in the
battery. The apparatus includes a device for sampling voltage and
current levels of each of the cells of the battery. Another device
compares the sampled voltage and current levels to voltage and
current limits to determine whether a cell has failed. If a cell
has failed, a switching mechanism produces an activation signal
which is conveyed to a switch associated with the failed cell. The
activation signal drives a heating element of the switch. The
heating element, which surrounds a fusible link of the switch,
melts the fusible link. The fused link congregates at the base of
the switch. In the case of a parallel network of cells, the fused
link forms an open circuit, while in the case of a series network
of cells, the fused link completes a circuit around the failed
cell. Consequently, the switch isolates the failed cell from the
good cells remaining in the battery.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent
upon reading the following detailed description and upon reference
to the drawings, in which:
FIG. 1 is a schematic view of a number of serial cells of a
battery.
FIG. 2 is a schematic view of a number of parallel cells of a
battery.
FIG. 3 is a schematic view of an apparatus for locating and
isolating failed cells in a battery, in accordance with the present
invention.
FIG. 4 is a cross-sectional view of an individual switch including
a fusible link which is surrounded by a heater element, in
accordance with the present invention.
FIG. 5 is a cross-sectional view of an individual switch with a
fusible link which has been melted and as a result has congregated
at the base of the switch, thus providing the switching action of
the invention.
FIG. 6 is a cross-sectional view of a switch which is connected
between two cells connected in series.
FIG. 7 is a schematic diagram of the use of switches in a parallel
circuit of cells, in accordance with the present invention.
FIG. 8 is a schematic diagram of the use of switches in a series
circuit of cells, in accordance with the present invention.
FIG. 9 is a schematic diagram of the use of switches in a
combination series and parallel circuit of cells, in accordance
with the present invention.
FIG. 10 is a cross-sectional view of a switch which may be used in
zero gravitational field applications, in accordance with the
present invention.
FIG. 11 is a cross-sectional view of the switch of FIG. 10 wherein
the heating element has been activated and the fusible link is
beginning to melt.
FIG. 12 is a cross-sectional view of the switch of FIG. 10 after
the fusible link has been melted.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to the drawings, wherein like components are designated
by like reference numerals in the various figures, attention is
initially directed to FIG. 1 which depicts a battery 10. The
battery 10 includes a number of cells 12 which are serially
connected by electrical conductors 14, as is known in the art. As
one can readily appreciate, if a cell, for instance 12A, fails and
results in an open element, the remaining cells in the series,
cells 12B, 12C, and 12D, will be electrically isolated.
Turning to FIG. 2, a battery 10 is depicted with a number of cells
12 which are connected in parallel by conductors 14, resistors 19
for current measurement, and buses 18. Again, if a cell, for
instance 12A, fails and represents a short, the remaining cells in
the series, cells 12B and 12C, would be subject to discharge
through the short-circuited cell. Consequently, the energy from
good cells 12B and 12C would be lost.
Thus, to prevent the open or short circuit effects of a failed cell
from disrupting the operation of the remaining good cells, the
failed cells must be quickly located and isolated. Current and
voltage information is required to determine the condition, or
health, of individual cells in the series of cells of FIG. 1. The
current in the series arrangement needs to be measured only at one
point in the series circuit. The voltage is preferably sampled
between cells so as to provide information about the condition of
each cell.
To obtain the required current and voltage information for the
series cell arrangement of FIG. 1, sampling wires 16 are provided.
Specifically, between each cell 12, a sampling wire 16 is coupled
to conductor 14.
Referring to FIG. 2, the parallel cell arrangement is depicted. The
individual cells 12 are interconnected by buses 18. Once again,
sampling wires 16 are utilized to collect data. In the parallel
configuration, one measurement of voltage suffices for the array
and one measurement of current must be made for each cell.
Turning now to FIG. 3, the parallel cell arrangement is depicted as
being connected with the other elements of the present invention.
Specifically, the sampling wires 16 and buses 18 are coupled to a
sampling device 20. Preferably, sampling device 20 is a sequential
sampling multiplexer. The output of sampling device 20 is coupled
to a logic circuit 33. Preferably, logic circuit 33 is a
microprocessor within a computer. A display 30 may be associated
with the logic circuit 33.
In a preferable embodiment of the invention, the multiplexer 20,
the logic circuit 33, and the power supply 23 are remotely located
from the battery. A remote location is required, for instance, with
the use of high temperature batteries, one example of which is the
sodiumsulfur battery which operates at a high, 350.degree. C.,
temperature.
As is known in the art, the logic circuit 33, or microprocessor,
includes a memory which can store operating instructions and
information regarding proper threshold voltage and current limits.
The operating instructions, as formulated by standard programming
techniques, compare the sampled current and voltage information, as
relayed from the sampling device 20, to threshold current and
voltage information.
The threshold information defines proper operating limits for the
battery 10. Naturally, these limits vary with the kind of battery
chemistry and complexity of the circuit for that battery. Current
or voltage information outside defined limits will indicate a
failing or failed cell.
After the logic circuit 33 has identified a failing or failed cell
12, it generates an activation signal, through standard techniques,
which is conveyed to power supply 23. Power supply 23 then supplies
a signal through activation wires 21. The signal from activation
wires 21 powers the heating element 22 of the switch 15 which is
associated with the failed or failing cell 12.
The switch 15 of the present invention is more fully appreciated
with reference to FIG. 4. FIG. 4 depicts a switch 15 connected to a
protruding cell conductor 14, associated with a cell 12. Switch 15
includes a housing 29. Within the housing 29 is a heating coil 22.
The heating coil 22 is coupled to a heater activation wire 21
through an electrical feed-through 31. The heating coil 22 is
supported by a heating coil support 25.
Cell conductor 14 is coupled to a fusible link 27. When an
energizing voltage is applied to coil 22 from power supply 23,
current passes through the coil 22 and begins to generate heat.
Eventually, the coil 22 generates sufficient heat to raise the
temperature of the fusible link 27. With additional heat, the
fusible link 27 will melt and flow toward the bottom of the switch
housing 29. As used herein, "bottom" is defined as that region of
the switch housing 29 closest to the gravitational center of the
earth, assumed to be the bottom of the page in all figures. The
fusible link 27 eventually forms a fused link, or stump, 32 at the
bottom of the switch housing 29, as seen in FIG. 5.
The heating coil 22 is thereafter disengaged and the stump 32 cools
to a solid state. The heater element may be disengaged after a
predetermined period of time or after relevant current or voltage
information has been measured by the logic circuit 33.
This final state is depicted in FIG. 5. In this final state, with
the cell conductor 14 disengaged from the fusible link 27, the
circuit will be in an open-circuit condition.
An embodiment for a series arrangement of cells is depicted in FIG.
6. In the figure, two cells 12A and 12B are connected in series
with a switch 15 positioned between them. A switch bypass conductor
35 is provided. This bypass 35 may be made of the same gauge and
material as the cell conductor 14, to be more fully described
herein. The bypass 35 is connected to cell conductor 14 at one end
and is connected to housing 29 at its other end. Consequently, the
bypass is usually an open element. However, when the fusible link
27 is melted, the fused link, or stump, 32 at the base of the
housing 29, forms an electrical connection with the bypass 35.
Thus, with the present invention, the electrical characteristics of
each cell 12 are sampled and measured. If the measurements reveal
that the cell 12A is not operating within given parameters or
limits, a signal is activated. This signal drives a heating element
22 which surrounds a fusible link 27. The heating element 22 melts
the fusible link 27. The melted portion congregates at the base of
the switch housing 29 and thus completes a circuit between the
housing and the conductor from cell 12B.
In a parallel arrangement of cells, the cell conductor 14 is
disengaged from the fusible link 27. As a result, an open circuit
is created and there is no danger of draining the remaining
parallel cells. In a series arrangement of cells, the fused link 32
forms a connection with a bypass conductor 35. As a result, a
bypass is created and there is no danger of isolating the remaining
series cells.
The functioning of a battery in accordance with the invention is
more fully appreciated with reference to FIGS. 7, 8, and 9. FIG. 7
depicts a battery 10 consisting of a parallel circuit of cells 12.
One switch, 15', of the parallel switches 15, has been activated in
order to isolate the associated cell from the circuit. The
remaining cells 12 contribute to the power and energy
characteristics of the battery.
The switching action for a switch 15 in a battery 10 with series
cells 12 is depicted in FIG. 8. As can be appreciated from the
figure, the switch 15 will complete a circuit around the failed
cell so that the remaining cells in the circuit may continue to
provide battery action.
It is not necessary that there be one switch for each cell of a
battery. The ratio of cells to switches can be greater than one.
This is illustrated in FIG. 9 where a battery 10 consisting of
cells 12 in a series-parallel network is schematically
illustrated.
The switches described thus far have been of a design suitable for
use in a gravity environment. Cells and batteries are also needed
in the zero gravitational (zero-g) environment of outer space. For
instance, batteries are used in satellites and in orbiting space
stations. These batteries will also benefit by having available
switches that can isolate a failing cell. The design and operation
of a switch of this invention that is intended for use in a zero-g
application is shown in FIGS. 10, 11, and 12.
FIG. 10 illustrates a cross-section of the switch of FIG. 4
modified so as to be suitable for operation in space. The switch
15A is modified by the addition of a zero-g wetting tube 38. The
action of the switch in this design depends upon the wetting of the
fused link metal 27 for various surfaces rather than upon the
presence of a gravity field; it is the function of the wetting tube
38 to provide the wettable surface in such a manner as to force the
molten link metal 27 to flow away from one end of the cell
conductor and to the base of the switch 15A where a connection to a
by-pass conductor 35 is made.
The wetting tube 38 is preferably constructed of a metal such that
its surface is wetted by the molten link metal. Preferably, the
wetting tube is configured to take advantage of the capillary rise
characteristics of a tube that is wetted by a liquid. Therefore,
the wetting tube 38 preferably consists of two sections. The first
region has a wide and constant diameter, the interior of this
region includes a coating 39 which makes the tube nonpreferentially
wetted by the molten link metal. The other section of the zero-g
wetting tube 38 is of a tapered design, varying in diameter from
wide at the middle of the fusible link area to a smaller diameter
at the end attached to the base of the housing 29.
In operation, the zero-g form of the switch 15A of this invention
receives activation power in the coil 22 of the switch 15A upon the
sensing of a failed or failing cell. The fusible link 27 begins to
melt and forms a sphere in accordance with the forces of surface
tension acting upon a liquid. This stage is illustrated in FIG. 11,
in which the fusible link 40 is shown in the process of melting and
forming a sphere.
Preferably, coil 22 is wound around coil support 25 in such a way
as to provide more heat at the ends of the fusible link as opposed
to the center, thus encouraging the fusible link to melt in a more
uniform manner along its length. As the metal of the fusible link
27 continues to melt, it tends to form a sphere, the sphere of
molten mass will increase until such time as the surface of the
molten metal touches the wetting tube at its narrowest part in the
middle of the fusible link 27. At that time, the forces of surface
tension will cause the metal to flow to the surface of the wetting
tube, and, in accordance with the forces described in capillary
rise equations, the liquid metal will be drawn to the more narrow
end of the wetting tube.
This behavior is further encouraged by the addition to the surface
of the wetting tube of a non-wettable coating 39 in the
larger-diameter, cylindrical, portion of the tube 38. After the
fused metal has flowed to its limit, it will have collected at the
most narrow end of the wetting tube 38. More particularly, the
fused metal will flow into chamber 44 of tube 38, where it makes an
electrical contact with bypass 35. Of course, in a parallel cell
arrangement, the chamber 44 would not be required; in a parallel
cell arrangement the fused metal 32A of the fusible link 27 is
disengaged from cell conductor 14 and therefore an open circuit is
created. The final stage of an activated zero-g switch in a series
cell arrangement is shown in FIG. 12.
Having provided a description of the operation of the present
invention, attention presently focuses upon some of the details of
a preferable embodiment of the invention.
Preferably, fuse housing 29 forms a hermetically sealed envelope.
The space within the housing 29 can be a vacuum or a dry and inert
gas. An appropriate sealing material must be used between the
housing 29 and external components such as cell conductor 14 and
activation wire 21. The sealing material must accommodate
differences in coefficients of thermal expansion. Appropriate seals
are well-known in the art.
The housing 29 may be made of borosilicate glass with seals to the
cell conductor 14 being made with graded seal glass. The housing 29
may be made of steel and insulated from the conductor 14 by use of
electrical feed-through connectors as is common in the electronic
cathode-ray tube technology employed in television tubes.
If the switch 15 is designed to operate with cells that normally
operate near room temperature, the fusible link 27 material can be
a low melting metal. Wood's metal is a metallic alloy which would
be suitable. In addition, the lead-metal alloys commonly used in
electrical fuses would be suitable.
Similarly, with ambient temperature batteries or with space power
supplies, the fusible link 27 may be made of any of a number of
fusible alloys having melting points ranging from 50 to 200 degrees
centigrade and made principally of bismuth, lead, and tin metals,
all of which are slow to oxidize at room temperature and which do
not tend to form structurally supportive oxide skins and thus may
be used directly as fusible links.
For the switches of this invention that are intended to be used
with high-temperature batteries, such as sodium:sulfur batteries,
the fusible link must be made of a metal whose melting point is
above the normal cell operating temperature. Zinc, aluminum, and
various alloys of such metals would be suitable.
Many metals tend to form a structurally strong oxide layer on the
surface of the metal. For instance, aluminum forms such a surface.
The presence of such a coating can interfere or stop the intended
flowing action of the fusible link 27. Accordingly, it is desirable
to remove that coating before sealing in the hermetically sealed
housing 29.
Preferably, a thin layer of a metal is formed over the aluminum to
protect the metal from oxidation while being processed into the
sealed switch enclosure. A non-active metal coating of lead or tin
metals may be used. The layer of metal may be deposited on the
aluminum by dipping the aluminum in a fused salt mixture including
lithium and potassium chlorides. Preferably, 5 weight percent of
lithium fluoride is also included in the mixture. The fluoride will
complex the surface oxide and expose fresh aluminum metal.
Alternatively, a commercial flux by the name of "ALUM-A-FLUX No.
1", manufactured by Forney Arc Welders, Fort Collins, Colo., has
been used. To assure that the fresh aluminum surface is coated with
metal the instant it is exposed by dissolution of the oxide,
preferably, 5 weight percent of either lead or tin oxide is added
to the mixture. The effect of adding a metal such as lead is to
allow a replacement reaction to proceed. The reducing potential of
the aluminum metal is adequate to quickly reduce the lead ion to
lead metal.
If the base metals such as lead or tin are not included in the
mixture, the aluminum conductor will be cleaned of the oxide
coating and, if protection is provided against significant
re-oxidation during assembly of the switch 15, the salt-coated wire
can itself be used in fabrication. However, the salt-coated wire
cannot be bent in handling and is not convenient to work with,
partly because of its hygroscopic nature. Accordingly, the cleaned
and base metal coated wire is a preferred embodiment.
In one embodiment of the present invention, aluminum wire of very
high purity (99.999%) has been used. This, however, would be too
costly for commercial use, accordingly, a form of aluminum (ASTM
1350-H19EC Aluminum) that is used in making electrical cable for
power lines of the electric utility has been used.
A nickel chromium alloy may be used for coil 22. Various forms of
alloys suitable for such use exist and are known by such trade
names as "Nichrome", "Tophet", or "Constantan". An adequate
insulating material for the heater coil support 2 is fused silica,
aluminum oxide, or various commercial forms of aluminum oxide based
ceramics such as "Morganite" or, for lower temperature
applications, borosilicate glasses, such as "Pyrex".
In one configuration of the present invention, 18 gauge "Nichrome"
wire was wound on a fused-silica support 25. Then, 25 to 50 watts
of heating power was applied to the coil 22. This level of power
was capable of raising the temperature of 00 gauge (nominally 1/4
inch diameter) aluminum wire used as the fusible link 27 to its
melting point within five minutes.
The zero-g wetting tube 38 may be made of tin metal or tin-plated
steels when used with the lower temperature forms of the switch.
The non-wetted portions of the surface may be made by proper
surface treatment such as chromating (for instance, "Parkarizing")
the surface, or plating copper on the metal and then converting the
copper to a sulfide or oxide. For the higher temperature forms of
the wetting tube, a steel base that is chromated for the
non-wetting surface, and, for the wettable surface, that is first
copper plated and then flashed with gold is preferred.
Preferably, the sampling device 20 and logic circuit 33 are
remotely located from the battery 10 and therefore remain at
ambient temperature regardless of the temperature of operation of
the battery of cells. In one form of the logic circuitry, the
following major equipment has been successfully used: Apple
Macintosh Computer; IOTech Model Mac 488B Mac to IEEE Interface;
Keithley Model 706 Scanner with ten switching cards (capable of
handling 200 cell sensing inputs); Keithley Model 199 digital
voltmeter; and interfacing software written in Microsoft
QuickBasic.
The foregoing descriptions of specific embodiments of the present
invention have been presented for purposes of illustration and
description. They are not intended to be exhaustive or to limit the
invention to the precise forms disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
application, to thereby enable others skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto and their equivalents.
* * * * *